I have a CNAME for
www.mydomain.com
I also have MX records for mydomain.com and *.mydomain.com
This is what happens if I try to add a CNAME for just mydomain.com or *.mydomain.com :
A validation error has occurred:
Since 'mydomain.com' has a mailserver defined using an MX record, you may not define the address for this name using a CNAME record. You should create an IP Address (A) record to define the address for this domain name instead, or remove the MX record first.
EDIT:
If I remove the MX record for the subdomain, it allows a CNAME for the subdomain....
So it clearly does not allow a CNAME for a domain if that domain has MX records setup ??